When mounting an electronic part on a printed circuit board, the electrodes of the electronic part are soldered to the electrodes of the printed circuit board. In this case, it is convenient to employ a method in which solder bumps are previously formed on the electrodes of the printed circuit board and the electrodes of the electronic part are soldered to the solder bumps by reflow soldering.
Formation of solder bumps are most typically performed by printing a solder paste using a mask followed by heating to melt the solder. However, with this printing method, as the number of electrodes on a circuit board increases with the electrodes becoming smaller and the electrode pitch becoming narrower, the yield decreases due to the occurrence of bridging and variation in the amount of solder, unavoidably resulting in an increase in manufacturing costs.
It is known to form solder bumps by a method in which solder balls are mounted on the electrodes of a circuit board using a jig and then the solder balls are melted. However, in this method, minute solder balls having a uniform size and a jig which is fabricated so as to match the electrode pattern on a circuit board are both expensive. In addition, mounting of minute solder balls on prescribed locations using a jig has the problem of poor reliability.
Recently, a number of methods of forming solder bumps using a solder transfer sheet have been proposed.
Below-identified Patent Document 1 proposes a transfer sheet in which depressions formed in a solder resist layer are filled with solder powder. The depressions in the transfer sheet have the same pattern as the portions to be soldered (electrodes) of a circuit board. When the solder transfer sheet is disposed so that the depressions oppose the electrodes of a circuit board and heated under pressure to melt the solder powder, solder bumps are formed on the electrodes of the circuit board. In this method, alignment of the transfer sheet is essential. In addition, because it is necessary to form depressions in the transfer sheet with a prescribed pattern, the manufacturing costs of the transfer sheet are high.
The method proposed in below-identified Patent Document 2 uses a transfer sheet having a supporting substrate on which solder particles adhere to form a monoparticle layer (a layer having the thickness corresponding to one particle) over its entire surface using an adhesive layer. This transfer sheet is disposed so that the solder particle layer faces the electrodes on a circuit board. The surface of the circuit board except locations of the electrodes is previously coated with a solder resist. Next, the circuit board having the transfer sheet mounted thereon is heated under pressure to melt the solder particles. The electrodes are wet by molten solder which adheres thereto, but the solder resist is not wet by molten solder. After the molten solder is solidified by cooling, the transfer sheet is peeled off the circuit board to obtain a circuit board having soldered bumps formed on the electrodes. The solder on the portions opposing the solder resist of the transfer sheet is solidified while adhering to the transfer sheet. The transfer sheet which is used has solder particles adhered to its entire surface, and as it does not have depressions, it can be easily manufactured at a low cost. In addition, the troublesome operation of alignment when disposing the transfer sheet on a circuit board becomes unnecessary.